1. Field of the Invention
The present invention relates to a porous composite metal oxide and a method of producing the same. More specifically, the present invention relates to a porous composite metal oxide, which is useful as an exhaust gas cleaning catalyst and the like for cleaning exhaust gas of HC, NOx, CO and the like, and to a method of producing the same.
2. Related Background Art
Presence of noxious gases, which are components capable of being present in environments surrounding the humans, and which are likely to affect the human bodies, have been drawing attention as problems in recent years. Development of an exhaust gas cleaning catalyst has been awaited, which can surely clean exhaust gas, for example, of its noxious components, including HC, NOx, CO and the like.
Against such a background, catalysts of various types for cleaning the exhaust gas have been developed. Methods of producing a porous body which has a large specific surface area even at a high temperature exceeding 1,200° C. have been described in Japanese Patent Application Laid-open Gazettes No. Hei. 6-199582 (hereinafter referred to as “Literature 1”) and No. Hei. 7-284672 (hereinafter referred to as “Literature 2”). In the case of Literature 1, alumina particles each with an average particle diameter of 10 nm to 100 nm, silica particles each with an average particle diameter of less than 50 nm, and the like are mixed up with one another. In the case of Literature 2, alumina particles, not less than 50 percent by mass of which have a particle diameter of not more than 100 nm, silica particles, not less than 50 percent by mass of which have a particle diameter of not more than 100 nm, and the like are mixed up with one another. In addition, Japanese Patent Application Laid-open Gazette No. Hei. 10-249198 (hereinafter referred to as “Literature 3”) has provided descriptions for a method of producing an exhaust gas cleaning catalyst. The catalyst realizes good durability of cleaning activity by mixing up ultra-fine particles, which bear a noble metal and which are 1 nm to 100 nm in average particle diameter, with other particles.
Nowadays, however, increasingly strict restrictions have been imposed on the noxious components in the exhaust gas. In this situation, exhaust gas cleaning catalysts need to meet more and more strict requirements for catalysts to be capable of surely cleaning such noxious components. Consequently, development of an exhaust gas cleaning catalyst has been awaited, which realizes a better durability against high-temperature, in other words, which can maintain a specific surface area and cleaning performance at a higher level even after a high-temperature durability test.